I am 70 and lift three days a week and ride or use my elliptical. The amount of muscle loss when I do not lift is dramatic and at this age it does not come back very easily. I would strongly suggest that you get into a three day a week lifting program that hits the major muscle groups and core. Your bone density is also at risk and no amount of bike riding will compensate for not doing resistance work. You might want to do higher reps, 12-15 for two sets with lighter weights as you build up your strength. I can not stress enough how difficult it is as you get older to retain much less build mass. Do what you wish but you will be very happy you did some weight training when you get in your 60's. Just my .02.
Frank.