Like you, I started training for health. I am 63, 6'1" and got my weight down from 240 to 195 lb.
I do at least 18 miles/day in one hour on a 6 mile circular road in a park. There are some hills. This I do with a light road bike.
On weekends I often do 100 miles/day on Wisconsin R to T. (unpaved with lots of bumps) I did the 100 miles in under 6 hr. with a Trek hybrid yesterday. My back and arms hurt from a past bike accident. The fully cushioned Trek with a sprung Brooks saddle protects me.
A 24 year old probably does not need all that protection and you will save weight and money. I wrote all of this for "backhurts." I believe that you will get a good bike for $800. The joy of riding goes up with the better components such as Shimano XT.
I hope this helps?
As to racing, the guys in my Metropolitan area beat the hell out of this old dog. But what the hell?