600 kcal a day at five years out is REALLY low. Maintenance for most people is somewhere in the 1200-1500 range (fortunately, for me it's more like 1700). Have you been eating 600 calories all along? Most of us just eat at that level the first year post-op. I can lose if my intake is 1500 or fewer calories - but of course that's going to be different for everyone.
Weight loss is supposedly 80% diet and 20% exercise, so losing is going to be more impacted by food choices than exercise (although exercise helps somewhat - and it's GREAT for general health). I wouldn't necessarily cut back on strength training. It doesn't burn many calories, and some people gain a few lbs when they first start (usually less than five lbs - and that's usually temporary), but in the long term it does increase your metabolism.
When I want to lose, I try to go back to the basics - Protein first, then vegetables, and then if I have room, maybe a small serving of fruit or whole-grain carb. I also figure out how many calories I'm averaging a day (and you can do this by tracking your intake for a few days, if you're not already doing that), and then cut back about 100 calories. If after a couple weeks nothing happens, then cut another 100. Rinse and repeat. But if you're truly only eating 600 kcal a day, I would NOT cut back from that. That's too low as it is.
are you getting enough protein? Drinking enough fluids?
also, keep in mind that if you're at or near a normal BMI, that last 20 lbs can be a BEAR to lose. I've been on a mission to lose 10 these last couple of months, and I can only manage to eke out about 2 lbs a month. That may be what you're dealing with as well..
