Pushing it Past the Limit

Over the last few weeks I have been pushing the the edges on the mountain bike. I did a "mellow" intermediate ride at the Land Locked forest last Thursday and then did a "hard beginners" ride there tonight.
Land Locked is a great place to ride, but I learned that my fitness level is still pretty poor on both rides.
The Monday night beginners ride has been great at building up my skills. There was nothing at Land Locked that I could not handle. Although there is a lot more at Land Locked to play on than there is in Belmont. That part made me feel good.
The Monday night rides are also good for building fitness, but not enough to really build it up to the point needed to do long rides without a lot of stops.
Back in June when I was riding my road bike to work a couple of times a week, I probably had a pretty good base. Now, after a month of only riding once a week, my old body has lost all of that base.
On the Tuesday night ride, I was lucky enough to find a two women who were looking for a slower pace. It gassed me pretty good and my bike computer told me I was overreaching. I felt pretty OK afterwards though - aside from my normal "Oh my god I'm gonna have a heart attack" anxiety.
Tonight though, I had no such luck. The ride leaders did not set a hard pace and did a great job making sure everyone was doing OK. I was doing fine for most of the ride, or so I thought. I knew I was pushing myself, but not too hard. Then I hit the wall.... We did this section called the milk crate and I nearly keeled over. Then there was more uphills...
At one point I just had to get off the bike and just rest awhile. The ride leader was really great and stuck with me and helped me stay positive. Some how I got back on and hobbled back to the cars.
My bike computer told me I was overreaching and congratulated me a new max heart rate of 194... I'm 50 and 220-50 is definitely lower than 194.... I wasn't having chest pain or dizziness or any more than normal shortness of breath, but I was wiped.
Somehow, I managed to get the bike back on the roof and after a little rest drove home. I am still in the "Oh my god I'm gonna have a heart attack" phase, but without any real symptoms.
It is just one more reminder that I am getting older and need to remember my limits. Oh and that I should exercise more than once a week.....
Fortunately, I'm vacationing next week and cannot be lured back to Land Locked. I'd like to say I have learned my lesson, but it was a ton of fun.