500 free improvement needed

Need suggestions for improving my 500 free. A little background: 52 year old male. been swimming again for about a year. up to 3500-4000 6 times a week. 5'10", 172 lbs (down from 195 when starting:)). Went to first masters meet last weekend. happy with times except 500. wanted to go 1:15,1:20,1:20,1:20,1:15. Went a 6:52 instead. 1:13, 1:26, 1:26, 1:27, 1:20. so about 43's per 50. swam the 1650 and went 44.5's per 50. (24:14). 50 free was 28secs. Even after swimming a year, pulls are faster than swims. Usual workout is 1650 warmup, 5x100s on 2 pull (~1:11s), 5x100s on 2 swim (~1:13s), 1x500 kick, 12x50's IM on 1:15, 10x25's no breathers free/fly. breath every 2 in the 500. breath 2/3 on the 1650 (no legs hardly). high exertion difference for only 1.5 second difference per 50. personal bests were 49 100 free, 4:59 500 free at age 20. worried a bit about heart rate max. even though 52 no problem hitting 180 on tough sets (if I shorten rest on 100s). Is that too high for 52? I thought 220-age should be about max. started swimming with a masters group 2 weeks ago but can only go on Saturdays. I'm guessing shorter rest on the 100's. (all distances are SCY) Thanks