I definitely don't think 28 is too old. as a matter of fact, it's actually the average age at most top schools. however, keep in mind that due to the nature of MBA recruiting , you're very unlikely to benefit from the extra year of work experience so i would encourage you to apply as soon as possible. I was 31 at matriculation (I graduated late at 25 due to a double major and an extended gap years + 6 years of work experience) and while I think age really isn't a factor (had plenty of classmates that were my age and even older), the amount of pre-MBA experience is. Consulting firms and banks, which hire the majority of MBAs, have all their new grads start as associate 1, regardless of pre-mba experience so it can definitely be a step-back for many. I had already done IBD and had no interest of going into consulting so it didn't matter in my case, but i have some classmates who really wanted to switch industry and will definitely be starting in a lesser role, including 2 ex PE guys who couldn't get a summer gig, freaked out and will soon be starting as IBD associates in the early 30s. i want to cry every time i think about them.
as for the gmat, i found the best way to get a high score (700+) if you're time constrained is to take 2 weeks off before the test and cram 14-16 hours a day. Most of my classmates did the same and we mostly agree that it's hard to study over a long period of time because you keep losing your form and forgetting the little trick to allow you to nail the tough questions.