The last few years, I've been mountain biking a lot throughout parts of northern and southern California. It's easy when your job allows you to travel to remote places. It gives me a chance to explore. Anyways, I've spent thousands of dollars reinventing my ride. I've gone from 26" wheels to 29" and in the process, I've sorted to making it lighter and heavier at the same time. I know...it sounds strange but it occurred to me that I've met people on the trail who have claimed to have done this and that but the one place they've not considered shaving the weight off was their mid-section.
Biking gives you a great cardio workout. The problem is, you have to challenge yourself. If you're only riding flats all the time or pavement or downhill exclusively, you never really push yourself to the next level. I've ridden many trails but I resorted to going on 3 specific trails in which I have time-trialed myself. Each time I go out there, I try to push it to the next level and shave a few minutes off of my ride time. There are sections where I'll have to go slower around tight bends or up and over rocks/boulders or fallen trees or through small ravines. Other times, I'll try make it up on the aggressive downhill or moderate flats.
But the one thing I learned that has helped me ride faster (at times) is having a workout buddy. That is motivation! Usually if you have someone there to root you on, you'll perform better and your results will be more noticeable.