There were no back-up routes. Basically, they went along as they saw fit.
You can find the mission rules on the Apollo Lunar Surface Journal. Unfortunately, only the one for Apollo 16 is complete with the mission rules for the LRV excursions (section 31), but presumably those for Apollo 15 and 17 are similar. Rule 31-1 is reproduced here:

LRV mobility rates will be determined in real time. These rates will be projected for maintaining the effective operational envelope and reassessing the traverse plan. The projected rates may be adjusted upward or downward from computed rates based on the qualitative judgement of:
A. Terrain differences
B. Losses in driving time attributable to surface observations
C. Unscheduled stops
Rule 31-31 and 31-32 establish the priorities for the EVA, and rule 31-33 made sure that those priorities were evaluated against (among others) the operational state of the LRV and the condition of the terrain:

Reassessment of the operational EVA plan will be done if any of the following conditions exists:
A. Behind timeline by more than 10 percent of EVA time remaining
B. Ahead of timeline by more than 15 minutes
C. Degradation of the LRV to such an extent that a minimum of 4 km/hr vehicle speed cannot be maintained or the planned traverse violates the redefined operational envelope