I started riding fixed after meeting a roadie friend of my dad's who cross-trains on a fixed gear and pushes an ungodly 53x15 or similar combo up and down the hills of Appalachia for miles and miles on end. Seemed like a great way to build strength while having new toys to play with. In the 8 months I've been riding fixed I've noticed several things with my riding. Foremost is that my legs are indeed much more powerful from pushing up the hills, which I could compare to spending hours in the gym doing squats (I ride a 44x17). However, I have also noticed that in only riding fixed these 8 months I've reduced my climbing speed and overall aerobic capacity. I think this can be attributed to pushing one gear up a hill, which builds power but not speed or aerobic capacity, and overall limits the speed of the whole ride when I have to slow down to get up a hill. I really noticed it today when I took my road bike out and did one of my normal routes and felt completely under conditioned for the speed of the ride.
Have other fixed riders noticed these results? Do you push yourself more riding fixed to compensate for these deficiencies? Mix up the training with doses of road or mtb to complement the benefits of fixed riding? From my other pursuits I know that you reap the most benefits from cross-training. That friend of my dadís obviously mixes it up and sees the benefits, but I was wondering what other people do as well.