working out and ttc

I got a treadmill for a Christmas gift which I am very excited about. However I'm ttc and haven't been very active . I would like to start working out but am a little afraid it won't be good to change my routine? I'm in the tww now and have worked out 3 times so far just 20 min at a time. How much is too much? Is it ok to increase it ? 
Thank you 😊