Exercise after open Abdominal Aortic Aneurysm Operation

I am a 72 year old male and am recovering from an open Triple A operation which took place 4 weeks ago. Before the operation I followed an exercise regime which included 15 minutes of floor exercises - press-ups/sit ups and back exercise then 5 minutes exercise bike and 5 minutes on Rowing machine followed by 1 hour beach walk with dog. Now, 4 weeks after the operation I am reduced to doing two 20 minute walks per day and finding that I am losing muscle tone and general fitness. My wound is the length of an A4 sheet of paper and healed well (as far as I can see on the outside) however I don't want to risk injury to inner muscle or tissue. Could anyone give advice on whether, or when, it would be safe to restart rowing machine and press ups/ sit ups etc. I would be most grateful for any help on this as the I received no information from hospital other than increasing short walks.