Hi Alan
You sound like you are in a similar position to myself. I weigh around 100kg and have signed up for my first century ride (Ride London) in July.
At the present moment I am combining gym work with a few short fastish rides each week. I have struggled to get out and ride recently though due to crashing my Giant and snapping the frame. That's all sorted now and the insurance have replaced my bike so I am going to follow one of the training plans on the Ride london website:
https://www.prudentialridelondon.co.uk/training/training-plans/
These may help you with formulating a plan to increase the mileage although you seem to be doing a good job so far.
I will be starting the intermediate plan I think to start with and see how it goes.
The main thing I have struggled with in the past when doing longer distances is fuelling on the bike. I can't seem to get the eating and drinking right at all. I don't like the energy drinks so only take squash with me. I can ride for 35-40 miles fine then crash into the wall and completely run out of energy. Something I really need to work on!
Good luck with your training.
Cheers Griff