Need cross training session to maintain endurance

I have an ankle injury and have been advised, after a few attempts at running on it, to give it four weeks rest. I am 1.5 weeks in and have been swimming around 3 times a week (2km each session), aqua running 2 or 3 times (intervals between 45-70 mins each session) and the gym (usually 20 mins x-trainer, 15-20 mins rower (mix of long and short intervals) plus weights and core week) a couple of times a week. However, I am worried I will lose endurance as none of these sessions are as long as my usual long run (18-25km). Any ideas for a cross training session that can replicate the long run? I did one aqua running session for two hours but now the outside pools are shut it is a bit busy for this.

An hours spinning on low resistence and intervals worked well for me. My workout would be 10 mins warm up , 40 mins on 1 minute spin as fast as I could and 1 minute semi fast and then 10 mins cool down. Shaved a minute a mile off my times when I returned to running too.