If you want to get stronger cycling, you need to cycle more. Swimming may help from a cardiovascular standpoint, but it won't give you the needed leg conditioning and strength for longer, hillier rides.

Regarding getting used to the road bike and clipless pedals, you have to do the activity to build familiarity with it. Road bikes are going to be more twitchy compared to a wide flatbar hybrid and the flatter back takes time to get accustomed to. A road bike is going to be faster and more efficient for nearly any kind of riding, but if you are a month out from the sportif, I think you'd be better served riding your hybrid.