I agree with the others who say that the whole "I'm tired/My legs hurt" whine tends to just be boredom.
We didn't have a car for the first 3 years we lived here (DD was 2.5-5.5) so walked and used public transport all the time.
We bought a car a year ago when she started school and I was pregnant.
During the summer, I did intend to walk the school run in the morning but I found it too rushed - even though I left in plenty of time I was worried about being late (I hate being late) so it wasn't pleasant for any of us. I didn't fancy walking in the afternoon as it was often above 30c and really "sticky" with it.
Now that it's autumn and the afternoons are cooler, we've decided to walk the afternoon school run. Did it yesterday and it was so much more relaxed than the morning one. I had put some HighSchoolMusical songs onto my mobile so we listened to them and DD hopped & skipped & jumped the whole way home. It's only about a 1.5km walk (just less than a mile I think?) and is pretty flat but everso slightly downhill on the way home.
Ultimately, I would like to walk both morning and afternoon. Once we're used to the afternoon walk and I'm less stressed in the morning we can add the morning walk in too.
I have some of the nicest chats with her on these walks - gives me a real insight into her school day and the way her mind ticks