I need lower gearing on my shimano 105 equipped madone 2.3 with compact double crank and 12-27 10 speed cassette. I was told I could put a mountain bike cassette on it. The reason for the lower gearing is that I haven't ridden in 20 years and I'm old (73), and live in an area with steep hills. The good is that I'm not overweight and I'm in good shape, just not strong enough yet for these hills. Also had a hip replacement 4 years ago so one leg is weaker than the other.

Is this as straightforward as I think, just new cassette, derailleur, and chain? Any advice (other than just take it to a bike shop) will be greatly appreciated. Thanks.