Plenty of people use a run/walk strategy from the start of ultras, and a significant minority use it for marathons (there is quite a well-known coach that promotes this approach, can't recall his name right now!). What flob said about pace & calories is related; below a certain pace, walking IS more efficient! The Romans found r/w was the fastest way to get across a country..
For you right now, a 38min 5k might be as hard as those marathons for the more experienced/able runners. So don't rule it out. I've used it when recovering from injury - and fully intend to again!
One suggestion - make it a controlled process e.g. 60seconds walk every km. That way you can maintain an even pace, and hopefully track your progress over time. (You don't really want to be struggling to finish, with no idea how long you walked for, how fast you were running, blah blah ... ) Just IMHO!