training for a century in 1.5 hour sessions

Saddle & food are likely the biggest concerns.

Is there a day when you can do two of these 1½ hour rides? I'd avoid the big hills, do long stretches into the wind (treat it as testing your resolve), ride a medium-hard pace for as long as you can hold it. Then a bit longer - grit your teeth, this last bit is what makes the difference to your condition.

I'd echo the comments about eating and drinking - worked for me when I did long rides

Don't underestimate the mental component. I've not had problems with the physical side of it, but telling your 'better' instinct to shut up, is easier said than done:

When I made the jump from just riding my 8.5 mile, each way, commute to doing metric and then imperial centuries the hardest thing was eating and drinking properly and then ensuring I was comfy in the saddle. It wasn't the distance that was a problem. Like you I did not set out for the best speed but found that the time I achieved was quite good, 6h 25m including the feed stops for the imperial. Just that bit of rub when I realized that I was 3min too slow for a gold. Just keep doing what your doing and ensure you eat and drink regularly as you go. Best of luck.

I would do high intensity sessions of 5 ,10 20 mins. Then it will be easier later in the ride. Long rides are more about drinking eating and being comfy. If you can ride 50 then you can do 100

Get out on the road, plot a nice interesting route, do it. If you make the route interesting. You won't even notice the distance.