Question 1145893
The problem as posted is perfectly clear for people who understand English and its nuances. 
The question posed is "How long should she plan to spend walking back?"
The question couldn't be clearer.
"How long" when referring to time (which is the reference here) means "How much time?"
Both tutors MathTherapy and josgarithmetic presented a good understanding of the problem and a proper approach to a solution.
A quicker and easier way to solve the problem might begin with tutor ikleyn's comment that her time walking back is 4 times her time running the distance of the race. 
So we can take the 4t +t=5t 
Now we take the 4 hours multiply by 4/5 to get 3.2 hours as both tutors MathTherapy and josgarithmetic found. Tutor josgarithmetic  went  a step further and converted .2 hr ( i.e. 1/5 hr) to 12 minutes.
So the simple solution"
8/2=4 times as fast 
4t+t=5t
4.0*4/5=3.2 hours=3 hr 12 minutes to walk back

Some tutors who don't understand English should be careful about complaining about perfectly clear problems. Sometimes, their answers are muddled English.

"The better English is, the worst is Mathematical meaning and Educational value of such "inventions". 
Precise and exact illustration.
Simply former English teacher tries to work as a mathematician.  Clear as a day. 
There are at least three big English errors in those four sentences!
1) Using superlative when comparative adjective is needed
2) Using an adverb when an adjective is called for.
3) Misquoting an English idiom.
I won't mention the incomplete sentence which may have been complete in the original context.