Leading up to major Mountain Biking events I'm a gym junky but first how's your knees. If alls good then good fat burning cardio type exercises is the way to go, I particularly like the stair master and the sitting down cycling type machines. Then again the rowing machines are good to, really good at raising the heart rate and getting us sucking in the air . Any type of exercise to get the heart beating harder is good, ask the gym to put a program together to suit your needs and tell them about the band. 60kgs in such a short period WOW you have done well.