I think the advice around time is a good one. Though you tend to take longer setting up for a ride.
Even if your bike is shit - though make sure it's safe - look at the time rather than the distance. It'll take a while till your ass can stand it.
I can't run anymore. So I ride. I was a sprinter, but I can ride 6 - 7 hours when I am in shape. I just got back from 2 1/2 hours on a mtb. It's the start of the season and I probably only rode 30 miles on the road.
It's very different depedent on whether you are on a road bike than a MTB. So I'd go with time rather than miles. That should balance your shit bike.
Running 90 sounds a lot to me - but I do ride 90 - 100 on the weekend. a 60 / 30, or 70 - 30. I average about 18+ mph on the longer ride as the area is hilly.
Stick with the time you have to train. Deciding on target miles may make it an impossible task. ALso they aren't relevant.
The numbers you got here are pretty accurate. It takes a lot of miles. But you can play with intensity.
Your challenge is that no matter how fit you are, riding big distance in groups takes skill. You won't have those skills. Especially with a shit bike.