LYNN – Participating in a triathlon usually isn’t a spur-of-the-moment decision because it requires getting into great physical shape.That’s where Sharon Dobbyn comes in.As fitness director at the Greater Lynn YMCA, Dobbyn has taken part in her share of triathlons and shows no indication of slowing down. On Tuesday, March 18, she will hold a meeting at 5:30 p.m. for those interested in training for such an event.The meeting in the YMCA main lobby at 20 Neptune Boulevard marks the start of a triathlon training course that continues through June 18. The class fee is $75 per month for YMCA members and $100 for non-members.Anyone interested in testing their stamina for an event that requires swimming, biking and running can sign up at the front desk or call Dobbyn at 781-581-3105, ext. 33.Dobbyn, of Nahant, participated in two triathlons in a single August weekend last year, one of which included a half-mile ocean swim, three-mile run and nine-mile bike ride. The run was on sand along Crane’s Beach in Ipswich. The second triathlon was Olympic-size, with a one-mile swim, 6.2-mile run and 26-mile bike ride.”I did it for fun,” she said. “June through September is triathlon season.”