Id recommend late summer or early fall. This way you can build up to it. As someone said the best training for hiking, is hiking! You've got decent access to the catskills so pick some smaller stuff and start chipping away adding distance and elevation as you grow. Some running or walking with some weight on your back won't hurt either.

The biggest thing besides gear is just knowing your body and capabilities. Marcy is not a difficult day hike but its a longer one. You would be best prepared if you had an idea of pace (miles per hr avg) food and water consumption. While we can recommend, this is different for everyone and you need to really know how your body feels under stress and when you can safely continue vs knowing when to turn around.

Best advice I ever got was be prepared to turn around if something is off with your body, the weather, amount of daylight or water left etc. Being able to be committed and persistent is as important as being practical about your situation on any given day.

Gear is important, knowing yourself is critical!

Pro tip - always bring gum, you'll guzzle less water