strongly suggest all longrunner do a full body check.
I check last summer find out there are irregularity in maximum heart rate graph when doing threadmill test. refer to specialist, do MRI test again (very expensive) but finaly find one of my heart blood vessel have 50% blockage!what a shock, I run 3 times a week and do many races.never think of having heart disease, I'm not quiting running but do with special caution and take medicine! so do not think such bad thing will not happen to you even if you're fit. even if you do sub3 sub 4 sub5.... BTW i finsihed FM in 4:47 unlucky number