This is a good thread. You are not alone, you know. Lots would like to do the things they missed out on when in their prime. Enduros and hare scrambles are great sports at all ages. Just look at the class structure. They have it by the age as well as by displacement! That should tell you something about who'll you be riding with...
I started riding when I came back from Vietnam. I was 21. Within a year I was riding enduros on a Yamaha 360. I can say I wasn't very good at woods riding, so that morphed into weekend motocross where I could learn the track and polish my skills through repitition. I still did an occasional hare scramble, but MX was something I could do regularly on a circuit. I did that until I was 32. I had my best success with a YZ125. For me, smaller was better and more fun.
My oldest son started riding at age 16. He quickly moved into road racing. He turned pro and did that for several years. He was hurt a lot so eventually he quit for good after a brief come back in 2008. Actually, real life got in the way.
Not six weeks ago he bought a 2012 KTM EXC-F 350. What a beautiful piece of architecture! It cost him $9200. That's a lot for a dirt bike... He bought that thing because he wanted to do some fun racing in a discipline that was new to him. Something he had to learn. What a great idea!
He entered his first enduro a couple weeks later as a 'Veteran C' rider (age 30). It was great! He loved it and had the best time. When I asked how the ride went, he said; "it would have been a lot easier on a 200".
He was referring to a KTM 200 2 stroke enduro bike. 200 lbs, great suspension and a sweet little motor.
Enduro's in the northeast require a street legal bike. The best tool is the KTM 200 or 300 two strokes, but they may not be practical for your wants. Hare scrambles are no problem for pure race bikes. My son bought the 350 4 stroke because he thought it would be close to the two strokes in performance, yet easy to register as a dual sport.
If you can do two strokes and want to go cheap, a 2000 or later Kaw KDX 200 is a good option for learning enduro's. If I were you, I'd go whole hog from the start, though.
PS: I always enjoyed my various mid-life crises. They produced great adventures, fun toys and lasting memories.
In fact, I'm trying to work up a late life crises right now! 2012 KTM 690R enroute...