lol i used to work at powerhouse gym in the states lol i know a little bit at least

man i weigh 248 pounds at 6ft tall and i race i dont get where this scrawny racer notion comes from.
especially for mountain biking upper body strength comes in handy.

I usually do high reps of 12-15 reps and about 8-9 sets per exercise but i have been training since high school so i wouldnt recommend starting out with that many sets, and i try to usually do 2-3 exercises for body part using a 3day on 1 day offf 3 day on cycle.

but to begin you need to start out slow with not as many sets and build up in weight and work on proper form and execution of the actual exercises, Also make sure to thoroughly warm up to sometimes i do a warm up set of something with much lighter weight to kind of warm up the muscle or muscle group then i start with higher weights.

make sure to stay hydrated to

and its good to cool down with some aerobics but thats a personal choice.