There are always fish to catch. Lakes provide fairly predictable hatches. Dry fly action starts in the morning, may come and go during mid-day may peak near dark. At other times the angler can probe the depths with sinking lines and add a third dimension to their sport. The river fisherman will find the transition to fishing lakes an easy one.
Most gear used in lake fishing is the same as that used on rivers.
Just swap those wading boots for fins and a pontoon boat and sample some new waters. You will also find lots of room for fishing and finding out just how far you CAN cast to a rising fish. Spending the day in a pontoon or belly boat takes a lot less physical exertion than hiking the broken terrain of a riverbed. It is only as aerobic as you make it. Just turn around and cast THAT direction for a while.