‘The task ahead of you is never greater than the strength within you” is a motivational saying (often associated with physical activities, such as running) that has been printed on many gift items, such as T-shirts and posters.  “The task ahead of you is never greater than the power behind you” was cited in print in 1989. The authorship of the saying is unknown.
 
 
19 September 1989, Winnipeg (Manitoba) Free Press, pg. 32, col. 8:
The task ahead of you is never greater than the power behind you.
(Obituary of Raymond Montieth.—ed.)
 
13 January 2005, Steinbach (Manitoba) Carillon, pg. 10, col. 6:
Betty’s Words of Wisdom: The task ahead of you is never greater than the strength within you.
 
RealChristianSingles.com
gamma58
Sun Feb 18, 2007 9:36 am
THOUGHT PROVOKING QUOTES
(...)
The task ahead of you is never greater than the power behind you.
 
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Iconic88
‏@Iconic88
“The task ahead of you is never greater than the power behind you.”
4:59 PM - 20 May 09
   
Google Books
One More Step
The 638 Best Quotes for the Runner:
Motivation for the Next Step!
By Randy L. Thurman
Bloomington, IN: iUniverse
2012
Pg. 154:
The task ahead of you is never greater than the strength within you. —Unknown
 
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Loggerhead Aquatics
‏@JCLALoggerhead
Oktoberfest Meet: The task ahead of you is never greater than the strength within you. #motivation
5:42 AM - 26 Oct 13