It sounds like you could start doing intervals.
I do my run intervals and my bike intervals on the same day. Cluster the hard work together. It was hard at first, but it works and it did not take me long to adjust.

I started out moderately with my interval training and stuck to a slow and steady increase of intensity and duration.
For example, if I was scheduled to do 4 repeats the first week, I stuck to four even if I felt like I could do more. Taking it slowly has allowed me to progress without injury(knock wood). Some days it was hard to hold back, but it was worth it.

So perhaps start with a warm -up, 30 second pick-ups with a min recovery , do 4 and then cool down. Or something along those lines.

I am sure the big guns will chime in with some good advice.

Ruth