If you find that riding becomes boring, I found good results by doing aerobic intervals on the bike. 2 - 3 min hard 3- 5 min normal to easy spinning.
Low impact, and strength benefits besides the cardio. Shorten the recovery and you get LT work, lengthen it and it is more VO2 :)
Sweet! I've been meaning to get into some interval-type bike stuff but haven't made it happen yet.