19 years older, retired for 27 years and did not start windsurfing until the 50's. Decided a worthwhile retirement was to learn to windsurf and wave sail. Slow and persistent, getting lessons at first.
Love to run the trails, wavesail on Pacific coast point breaks of Baja, Mex. and Chile. Chile side-onshore the most challenging, however. Learning to kite now in a lagoon in Canada, and also southern Baja, doing turns reliably. Must start to practice toeside, then switch to a directional board. Do not plan to stop windsurfing; rather, to do both. My quiver has 4.2, 4.7, 5.3 and find myself under and over powered a lot. Gear stashed in three locations. Also use a SUP surf a few times each year in an ideal location for that.
Runners have to deal regularly with their physical limits to avoid bench time and this helped me pace myself in new sports. WS seasons for me are spring and part of summer in northern AND southern hemispheres.
Some things that have helped:
Have modest expectations after a break between austral and boreal seasons, doing shorter sessions with recovery days at first, and letting some days with good conditions go by for recovery. Rest or go for an easy run, do pushups, ab curls, etc. Using exercise bands, but not enough.
Maintain flexibility with stretching and do thorough warmups before pushing.
Run hilly trails for endurance, for the legs and lungs. Have been doing running circuits on grass or track or hilly trail: few laps warmup slow, some backwards running, skip rope for a bit after each lap, do a few pushups, maybe some pullups, and some strides if well warmed up. Hill repeats are also great if thoroughly warmed up, doing the first few cautiously. Did very little competitive sports, outside of some mediocre tennis in school, and only informal stuff when younger so my joints have been good. Bodyweight squats are helpful, two legged deep and one legged shallow squats.
Diet, vegan for the last 12 years, now eating fish infrequently. Avoiding fruit juices, meats, highly processed foods and taking B12, D3, etc.
Keep foot straps wide open for ease of separation when getting washed in surf--feet are nearer the center of the board for leverage. Impact injury time has come from staying with the gear until the last minute. If staying with the gear, never let go of the boom. Most wave sessions, I have to swim for my gear at least once. On some intimidating days, either play it safe or wait until conditions are less radical. Learning how to fall on turns helps a lot.
Backing off when injured, to recover, whether it is from running or windsurfing or .... Having breaks during the year to cross train. During working years would join road bike groups on weekends and run during the week. Did many all day cycles in the hills, one running marathon and more shorter events and one tri.
Windsurfing develops the lats, so after a season, pull-ups are possible. Conversely, pull-ups would be great in the off season.
Learn how to do my own physical therapy to prevent overuse injury, compensate for lots of inactivity while online, e.g., and recover from injury or incipient injury. Be attuned to points of vulnerability. Sometime, just a stretch, or fixing a muscle imbalance, or an insert in shoe or bootie. Good posture is a challenge right now, suggests core work.
Stay covered up to avoid UV damage to pale skin and visit dermatologist regularly. Wear protective equipment for head, eyes, ribs, cold, etc.