i didn't get into serious distance running until college. i finished 0/4 track and 3/5 XC seasons in good health, was an injury mess (3 knee surgeries, never more than 6 months of consistent training, etc). This was at a good D3 program but i was nowhere near your level of talent (4:28 mile, 25:16 8K).
5 years later & nearing the finish of my PhD program i decided to get serious again (stayed in shape, but i'd call it "running" and not "training") because 10 years from now i didn't want to look back with regret & wonder, "damn, how fast could i have ran if i trained hard & stayed healthy?". last fall marked my first full year of injury-free training EVER, a year in which i ran PRs at 800, mile, 5K, 10K, and a 1:08 half debut. gonna tear up the track one more spring, then turn the focus to chasing an OT qualifier in the HM/M. i just turned 29.
if you're thinking is "28 is too old to run fast", stop. it's your prime. you can improve well into your 30s. people get LAZY before they get old...most 28 year old guys are out of shape not because they are 'old', it's because they are so busy with career, wife, kids, social life, etc. that they don't make the time to take care of themselves and/or commit to training. which is fine if it isn't a priority for them.
so if you make up your mind and say "i am going to do this, i am going to run times x,y,z", you can. age is not holding you back, it's whether or not you are willing to put in 2,3,4+ years of consistent, smart training. if you make training a priority in life, you will run better than before. it's up to you - good luck!