if you get a heart rate monitor you will be better able to judge your physical conditioning than by just the number of miles ridden or weight.
my doctor kept telling me to lose 10 pounds. I started doing things to get in better shape, hoping to lose weight. I went down 4 pant sizes(38-34), 2 shirt sizes(3x-1x)went from a keg to a 1 pack in the belly and went back to the doctor just knowing Id be down to about 180. WRONG, I was 7 pounds heavier but my bp was down, hr was down, colesterol was down, bmi was down.
I am 20 pounds over my doctors advised weight but am in excellent physical shape due to gaining muscle as well as endurace.
the monitor has shown the gains more than anything. I am 42 and can get my hr up to 200 for over 1/2 hour before getting worn out( I found this out last summer at Duncan Hollow trail). I stay in the 170s or higher for my entire rides and ride for 1.5 to 2 hours at a clip without stopping and have ridden for over 6 hours at that rate more than twice in the last year
oh and being over 30 is not the time to give up on the fun. I ride aggresive xc and very technical xc.
good luck on the training and MOST important is to enjoy what you are doing, the results will come with time. if you like the ride you are doing, keep doing it.