The usual advice for post surgery activity is to refrain from any exertion for at least six weeks and then only gradually introduce yourself to gently increasing exercise. Impatience in the early stages of recovery can be a long-term disaster. It would probably pay to discuss your feelings and aspirations with an ostomy nurse before going to the gym. I found a useful article on post-op stuff on the internet so will reproduce it below:
AFTER THE PROCEDURE.
You may need to stay in the hospital for several days after the surgery. When you go home, make sure to follow all instructions for medicines, pain control, diet, activity, and wound care.
If you have a colostomy after surgery, you may need to work with an ostomy nurse or enterostomal therapist. Make sure to keep all your follow-up appointments. He or she will show you how to use the ostomy pouch, deal with side effects such as skin irritation, and resume an active life. With proper care, the pouch cannot be seen under your clothing, and people will not even know you are wearing it.
Some other common instructions after proctectomy may include:
Get up and walk as much as possible soon after surgery.
Gradually resume your normal activities. This may take a few weeks. Ask your healthcare providers when you can resume bathing, driving, working, and sexual activity. Don’t do heavy lifting for 6 weeks.
Watch your wounds for any sign of infection. This may include swelling, redness, bleeding, or fluid leaking.
When to call your healthcare provider
Call your healthcare provider if you have any of the below:
Fever of 100.4°F (38°C) or higher, or as directed by your healthcare provider
Swelling, redness, bleeding, or fluid leaking from the incision
Pain that gets worse
Shortness of breath