What was the hardest personal goal you’ve set for yourself?

The hardest goal I set myself
I never did achieve
A marathon in sub 3 hours
Was at one time my aim.
I took it very seriously
Would follow six month plans
Build my weekly mileage up
To sixty miles a week.
I’d study all the training plans
For stamina and speed
Watching what I had to eat
And getting proper sleep.
I had a few attempts at it
And mostly did OK
But 3.15 the best for me
And sadly that won’t do.
I gave up running marathons
When I couldn’t beat four hours
I have gentler running aims
Now I’m seventy two.
I still like to push myself
But have more modest aims
I still aim for 8 minute miles
But now for just 5k.