Training plan for 10 mile circuit

Push hard, it's the only way. You'll soon be rolling those 10 milers, with ease.

"Make every ride count" I use my daily commute as a training ride, pushing at every hill and endeavoring to go better / faster than the last time, obviously this will peak but then its about consistently achieving the same time. My commute is about 8.5 miles each way so not too dissimilar to the OP's. When I then jumped to metric and imperial centuries it wasn't as hard as I thought. I am still improving on time. My first 100k took me 4h 15m but now I have that down to 3h 47m with no concerted training program other than the daily commute and making each and every ride count. I would also agree about eating less crap it's certainly helped me along the way, as has learning to hydrate and eat properly on the big rides.

Ride your bike hard. Make it hurt (in a good way). If you're dedicated then buy a turbo trainer and visit the pain cave when you can't ride.

Time your 10 mile loop. Aim to go faster - not every time, but over time. Spin a slightly higher cadence than normal some days. Other times engage a bigger gear, focus on pushing through the pedals from your hips, not your knees.

Eat less crap and more healthy, unprocessed food. Drink more water and less sugary / sugar-free sweetened drinks. Never consume so-called diet products, they are junk.

Don't waste all evening on the internet or watching TV. Get a good night's sleep.

First few weeks I would just ride the route s as often as possible. Then when riding try holding a gear and speed going up the small inclines, repeat as many times as you can handle. This will lead you to an increased speed generally. In a few months you will be riding much faster attacking all the hills and carrying speed over the crests. Once you find these local courses easy extend the time riding and repeat same above.

It is quite amazing how quick you can go from 10 miles to 50+ miles in a short period

Like you I am heavily time constrained and take the following approach:

During the week, shorter rides which are a mix of:
Strength work - hill reps or riding higher gear/lower cadence on a route with some climbing
Threshold sessions - near constant efforts with 60-70% in threshold heart rate zone
Intervals - either turbo (hated) or a route with a mix of hills and flat sections where I attack the hills.

During the week I end up doing mostly threshold sessions as that's the kind of riding I have near work. Weekends are longer to work on endurance, done early in the morning and importantly before eating anything more than a banana. A mix of tempo/threshold riding with 2-3 efforts on hillier bits and 2hrs +.

Build up gradually over time but look to get 3-4 sessions in a week I suggest.