I might get called out for this being the kookiest technique ever, but it’s worked for me! My view is that the best way to get into surfing shape is by surfing, but sometimes the ocean isn’t within reach. So, grab a medicine ball and a shitty beater board that you don’t care about. Put the board on top of the medicine ball so that it’s balancing on the ball (preferably a heavily glassed log so it doesn’t snap).. Grab two 5lb weights (1 in each hand). Lay on the board just like you’re surfing, and paddle for as long as you can with the weights in your hands. Balancing the board on the ball will work your core just like when you’re paddling, and swinging your arms while holding the weights does a damn good job of mimicking actually water resistance. Yes I’m a kook, but it works for me!