Well, good for your Doctor!!!! :D
And welcome to the forums. There is a ton of information here, and some really good people willing to help.
Did your doctor limit you to 20 mins, or as is usually the case, is that the minimum you should try to get. Obviously, if he limited you then stay there. But if that's a [i:abc352d406]minimum[/i:abc352d406] you should strive for, then 45 is not contraindicated.