If your goal is to improve your cardio and have it carry-over into your riding, I would recomend mountain bike riding or running on varible terrain (hills) or incline on treadmill. Keep in mind that when we ride our motorcycles it involves quite a bit of eccentric muscle activity (contraction as muscle lengthens) sort of additional suspension. So off-saddle conditioning should try to incorporate this activity since it is specific. The benefit of mountain biking (when standing and absorbing) and running to a lesser degree is that it involves eccentric activity. Stationary bikes do not, and is really not a weight bearing activity since your are sitting. Low intensity running can actually weaken your legs. Mix in some intervals or hill work after building up your base mileage. If you run, you should definitely supplement with lower extremity weight training. Once again, stay away from exercises were your feet are in the air (leg extensions, curls etc.) Squats on a rack or Smith machine and lunges are good ways to reproduce leg activity when riding. Of course the assumption of good technique and high rep, moderate weight is desired. Rope jumping and some plyometric exercises can also be beneficial. If you mountain bike, hit the trails!! I have friends that say they are going mountain bike riding and ride 20 miles on a flat "nature trail" and their butt never leaves the seat. Find some hills and bumps to force your self to stand and weight bear through your legs. This will transfer well to your motorcycle. I have nothing against stationary bikes, elliptical trainers, swimming or anything else that floats your boat and your joints can tolerate, but if your goal is for improved fittness combined with carry-over to the motorcycle there are better ways than others. If you are starting from scratch any aerobic exercise will likely help your riding. A good thought process is get healthy first (just get out there and exercise), then get fit second (try to individualize your work out for improve performance - fine tune) and then get specific for your activity, be it motorcycles or anything else. You will enjoy riding so much more when you are in shape!!
Good Luck, Keith.
here's another view....stair stepper, stair hard!..not a little walk, i find my legs get much more of a pump after 20min stair vs running, plus ya get a good glute pump. after the stairs it's off to the free weights to keep the cardio effect up, followed by the naut abductor. btw, after getting my bike and riding hard(relative term) i found that my work out up to that point was not effective for my riding needs, especialy upper back and forearms. i've totally eliminated my bench press:( .for alot more bent over rows with a free bar. damn...now if i could just ride!:think oh yeh, ps. don't make the mistake off assuming that alot of the periphrial muscles used for balance don't come in on the stair, don't be like one of those morons that leans over the top of the display and supports your weight while pumping you legs a total of 2", stand up straight and go hard, long strides.