electronic overdrives act like a 5/6th gear, and are used for cruising (ie motorway driving) If you engage the overdrive at to low a speed, it can put too much pressure on the overdrive and cause it to burn out.
You need not be on a "long run", as Toureman says , regard it as a higher gear. However, just as using say, 4th gear when your speed or a hill means you should be in 3rd, then using the o/d will burn more fuel.