I've found the estimated times to be almost laughably bad, and worse the longer the distance.
Right now, mine estimates:
- 5k 17:22
- 10k 36:02
- Half 1:19:38
- Marathon 2:46:25
My 5k PB is 18:48, 10k is 38:55, and I've only done one marathon at 3:14:24. But right now? My goal is to do a sub-18 5k by the end of the year, and that is far from a sure thing. I've no hope of doing it today, and I doubt I could even complete a marathon (at least in any remotely reasonable time) today.
So no, I don't put a lot of stock in it.