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It’s never too late – i.e. you’re never too old or out of shape – to ride! I’ve come across guys who are in their 70’s riding very demanding trails. You just need to give your body a chance to adapt.
When you say “I can’t emphasize how much it hurts to sit on the bike” it makes me think one of two things: either you have saddle sores and/or your glutes are very sore. The former is a tissue issue and is best addressed by wearing a chamois under your shorts (or at least an anti-chafing cream). The latter is a muscle issue which will improve over time but in the short term I’d suggest taking a couple of Advil. It’s important to address this issue as riding comfort is critical. It’s one thing to feel discomfort on a big climb or riding a tech section – that’s expected – but it’s entirely something else to have discomfort just getting on the bike!
Riding is great but it’s most enjoyable as part of a (generally) healthy lifestyle. As @bikenerd stated start slow (think easier effort and shorter distances) and work you way to slightly harder/longer rides as you become more comfortable. I’d higher recommend tracking your rides with your phone as it will be gratifying to look back and actually see the progress you’re making.
I am also a big proponent of cross-training. It doesn’t having to be complicated but going for walks/hikes – especially in hilly areas – will help build your fitness. And I think everyone should do strength training at least twice a week. Doing 3-4 sets of squats (using just your bodyweight) and deadlifts (with light dumbbells) will work wonders within a month.
Let us know how it goes.