Originally Posted by OZ Puncher
Do the exact thing you do for running. A medium pace for 1 km, sprint for 500 m. Repeat till you get about 4-5 sprints in. Less stress on your joints in cycling. I'd also suggest swimming.
Do you have designated bike paths in your area? If so, I'd suggest using them to get a consistent workout as opposed to riding on city streets and all the traffic. Where I live, I'm fortunate that our city is becoming more "bike friendly" and many bike paths are being constructed.
There is one on top of the levee of the Mississippi River that runs just under 24 miles of 10' wide blacktop. When I hit it, I try to keep my time just under 3 hours for an out and back ride of about 48 miles. That averages out to about 16 mph which isn't too bad considering I'm pushing 60, ride solo and have nobody to draft off of.
This type riding won't benefit you as much as sprint work to get your heart rate kicking in high gear, but it works for me and my bad knees and no desire to box at my age.