Decent rides starting from North London going further North?

 http://player.bfi.org.uk/film/watch-cyclists-special-1956/

You're very welcome. 

https://hexcycling.com/rides/

Check the route of the Dunwich Dynamo, a lovely ride that starts by heading east on Lee Bridge Road, then turns northeast to run up through Epping Forest and into the Green Belt. You'll easily find it on the usual websites. Loads of short hills in Hertfordshire too, between Ware and Royston is lovely cycling country.

If you're heading south, check out the climb guides on www.broleur.com - you can find other lists of climbs around London (e.g. Simon Warren's book) but these are free!

You could always try riding with Islington CC (www.islington.cc) - they have a number of rides on midweek evenings (30 - 40 miles) and weekends.   

Sack off North London and head down South!

I grew up in the Chilterns, there are some lovely rides around there - but I always start from my parents place in Gt Missenden so have never ridden in that direction from town.

I live in South London now, in a place called Sutton.  If you are going to consider getting on a train anyway, then don't discount heading South.  Sutton is 50ish minutes direct from St Pancreas or 25 minutes from Victoria - from here you have about 1-2 miles of urban riding to do before you are out in the countryside.

I've got numerous regular routes I do varying from 30 miles (box hill Olympic loop) to 75 miles which take in most of the big hills in the Dorking area.  If you cut through some of the smaller villages you completely avoid all the big roads - making it a pleasant place to ride.

If you want some routes I'll share some Strava links.

North London is a bit of a traffic quagmire that seems to go on for ever - and for little reward.  I would have thought it better to head south, especially if you want hills.  If you set off early enough there's little traffic in the City heading south towards Kent.  I use the A205 to Bromley then head towards Biggin Hill, Toys Hill and then the Kent Weald and Ashdown Forest.  If you don't fancy getting up that early then you can jump on the Overground and get off somewhere south.  There's a few decent cafes in Crystal Palace that cater for cyclists too: St Germain and that arriviste Giant bike-shop. 

Once you get through Watford take a left just before The Grove and you are straight into the Herts countryside and Bucks Hill. The roads here are mostly quiet and undulating. Soon after you are into The Chilterns and places like Chesham and Tring, lots of good riding around there as has been said.

I plotted a route for someone from Borehamwood to Stevenage last week, once you get past Barnet you're all good.

This is a rolling route, approx 45ft/mile, maximum 10% (aside from highgate hill)

A1/A1000 toward Barnet, carry on to Potters Bar, fork right onto B158 (Essendon/Cole Grn)

Swing around roundabout on A414 1st exit onto B1000 (Bramfield), Datchworth, Woolmer Green, left onto B197 toward Welwyn under A1.

Welwyn Bypass Rd B197 (Disgwell Hill/Gt North rd), R to Coopers Grn La, L into Oaklands La, cross A414 to Colney Heath.

Swanland rd, L into Mutton La, Baker st, under M25, L into St.Albans rd back to Barnet then back along A1000 to islington.

works out roughly 60 miles and 2,400ft elevation. Try bikehike and some basic Audax style notes, you don't need a GPS that's for sure.

HTH

Join a club and ask them for their routes, club members will have plenty and they'll even show them to you if you turn up to a club run

Ashwell and Morden only has (until late next year) 1 train per hour all week. Baldock has 2tph and is very close by, easy to get out of.

Even Stevenage is a decent starting point. No really. Little bit of clever route planning, use the fantastic cycle network to get to the edge of town and bobs your uncle, Hertfodshire in all directions, head west towards Bucks, north into Bedfordshire, north east into Cambridgeshire and stirke to the east for Essex.

Also: Lea Valley. Follow NCN1 (it's mainly OK) out of London into the Herts/Essex borderlands. Probably best to avoid after rain, but a few dry days and the unsurfaced bits are about 96% hard dry dirt, smoother than many tarmac roads. OpenCycleMap and CycleStreets (there's an app version) are handy

+1 for getting the train out if you don't fancy riding through Finchley and Barnet. Although it's not that bad, TBH. Once you're outside the M25 there are lots of nice rides - my usual routes take me throut Ayot St Lawrence, Kimpton, St Pauls Walden, up towards Hitchin. Or go to the east side of the A1, Tewin, Bramfield, up to Bennington etc - lots of nice little country lanes to get lost in, and nicely undulating.

I met a guy who lived in Kilburn and used to go out riding in the Chilterns. I think he said he used to head out there via Wembley/Stanmore/Watford direction. There are quiet roads heading that way if you look hard enough! Or let the train take the strain, as others have said.

How about jumping on a train & getting out to somewhere pleasant (on the right side of the M25).  If your not a huge distance from Islington you could get Ashwell (and Morden) [Hertforshire/Cambridgeshire] in 38 Mins from Kings Cross - From there head off towards Essex or somewhere south of Cambridge and get a train back from one of the lines over in that direction.

It might cost a bit but would be prefrable to miles of urban ick - Just make sure that the trains are actually running on that day (and look at getting a 1/3rd off railcard if your likely to do it frequently) 

p.s - The Ordnance Survey 1:50,000 maps are your route planning friend.

Depends what your range is like. Best riding in my opinion is across the Chiltern hills, that would be a good 80m+ round trip to somewhere like Princes Risborough or Wendover and you'd have a large urban sprawl to cross to get there.

better to head north east towards epping forest if you want to find a few short but steep-ish hills