ant to get gym fit for free? The first 50 Boston residents, with I.D., who arrive at Northeastern's Marino Recreation Center every weekday after its 5 a.m. opening time get to work out in the gleaming fitness center at no charge. This isn't just your average gym. There are stairclimbers, treadmills, upright and recumbent exercise bikes, cross-country ski machines, rowing ergometers, elliptical climbers, three basketball courts, 42 free-weight stations, 40 weight machines, a three-lane suspended track, a Treadwall that simulates rock climbing, men's and women's saunas, and an atrium with a café. But you'll have to get up early to get in. Or take the SAT again.