I'm somewhat younger than yourself and during my time with the temp loop ileostomy I had the same concerns; I think I took up to 6 Imodium per day which appears to make little difference.
Since takedown, my output has been completely different, although loose, acidic and sometimes pure liquid initially; more often than not, it's of a much thicker consistency.
4 year since takedown, I rarely take the Imodium although I should and my output is loose, it's still hell of a lot thicker than what it was with the loop illestomy.
No bed time accidents or toilet visits either other than to pee, although I do have episode of leakage during the day, sometimes heavy.
I think any issues in regard to incontenience, will more than likely be associated with age rather than the J pouch.