Swimming. I recommend swimming.
Also here’s something you can do if you dare:
Carry a rucksack, pack it with 45lbs (Not including water weight) worth of shit (pack appropriately so it doesn’t shift all over you) and just go walking for the following durations over time.
Week 1-3 (2x/week)
4 miles @ 4mph pace (60min or less)
Week 4-6 (3x/week)
6 miles @ 4mph (90min or less)
Week 7-9 (2x/week)
10 miles @ 4mph (2.5hr or less)
Week 10-12 (1x/week)
12 miles @ 4mph (3hr or less)
remember to have the weight secure around your hips and swing those fuckers like your life depends on it keeping your core tight as possible. This is an easy way to focus on total body in a compound movement with some purpose that will really build your core, however - abs are indeed made (shown) in the kitchen.
best of luck.