"She ran the marathon in less than five hours."
Translation:Zij liep de marathon in minder dan vijf uur.
If you replace uren by uur it's correct. We don't use the plural of uur to indicate a time length, only when separate instances of an hour are meant and for quantifiers (when the exact amount is not known), e.g.
- de eerste twee uren gingen goed = the first two hours went well (two instances of an hour are meant)
- de eerste twee uur ging goed = the first two hours went well (a time length of two hours is meant)
- het ging twee uur goed = it went well for two hours (time length)
- honderden uren = hundreds of hours (quantifier)
- enkele uren = some hours (quantifier)
Kwartier (quarter/15 minutes) and jaar (year) are handled in the same way as uur, these 3 are the exceptions. All the other time units (second, minute, day, century, millennium, etc.) use the plural form whenever there is more than one.