Once pro runners reach their limit (Kenenisa for example, reached it when he ran a 26:48 10k), they are almost always on a gradual, yet constant rate of decline.
For all the work they put in, from the 20+ mile long runs to the 100-120 miles per week they put in, their progress halts.
Do you ever find it sad they make such efforts day in day out, yet they work hard only to stem the speed of their decline?