There is a good article on Active.com about runners cross training with Stand Up Paddling
Whether you are training for a marathon or tri-athalon or just want to keep in shape for the next ski season Stand Up Paddleboarding will keep you in shape. Paddleboarding is also great cross training for snowboarding or cross country skiing. You can go at your own pace or sign up for a SUP regatta and race your SUP. There are regattas through out the US to participate in.