As the World Series approaches (Go Nats!), today's puzzle has a sandlot feel. The revealer tells us what's going on at 36A: Bases loaded ... or a hint to the contents of 17-, 26-, 44- and 56-Across (THREEON). It turns out that the four theme answers have the letters ON repeated three times within them.
I've never heard the phrase KNOWONESONIONS, but I definitely have to use it on a regular basis from now on. I suppose the folks across the pond would be familiar with the LONDONMARATHON - round these parts we're partial to the Boston version (and to a lesser degree, the one that wends through all five boroughs of New York City). But the other two answers are very strong, especially BLONDEONBLONDE (although do you lose points for the word "on" being used? I say nay).
There's a bit of extra sportese sprinkled through the grid, including ESPN, HANGSTEN, ONENIL, and TURKEYTROT, if you will. You definitely would get TANLINES if you're out surfing, also...
I'll note my dislike of abbreviated answers being clued as parts of abbreviations. Today's example comes at 20A: The "E" of E.S.L.: Abbr. (ENG). It's so tortured. But that's the only answer I minded, so that's a good ratio. I also liked the grid layout, which flowed well despite having four very long theme answers and a revealer.