It's nice to use Lagat as an example but he is really exceptional.
Geb would be a better barometer.
He ran on the track in 4 Olympics but did not medal in the last two even though he kept good fitness through the years.
In 2008, Geb set a WR in the marathon but could not medal on the track.
In 2004 he fell short of an Olympic medal even though he earned a WC Silver in 2003.
Bekele will be 30 this summer.
Considering he ran the world lead last year, I would say it is safe to assume he will be a contender this year.
4 years from now?
Only time will tell. The odds are against him but I think it is mostly up to him and how much he wants it.
We may see an El G move where he puts everything into this year and tries to finish on top and then walk away.