Well I suppose this is a forced rest, sounds like you need it. Unfortunately right now maintaining your cardiovascular fitness will have to take a back seat to recovery from surgery that whose recovery does need to take priority. Any abdominal surgery takes a fair amount of time to repair and the body doesn't like things touching the internal organs and this can be problematic during the early recovery phase. As a result you need to take it easy and remember that a forced rest will bring you back stronger in the medium and long term. Think of it like you just finished the Olympics, then had a down time for a few months knowing that when you return you'll just work on some basic endurance.
Perhaps spend this time working out a better training schedule, better diet, perhaps a goal at the end of the next training phase - adventure holiday etc. This is what you have the time to do right now so don't think about maintaining the CV fitness you had as you can't. You steady workload was to go prior to maintain that so it will drop off in any case. Best of luck
