It's a little unclear what you're looking for. Training for long distance isn't really enough to make a podcast series out of. Fast Talk is likely the best training podcast out there but there's a lot of bull**** in some of their episodes.

Not sure if you're a racing fan, but:
Velonews: pro cycling focused but the guys are genuinely funny and knowledgeable. Also starting to delve into cross next year.
The Cycling Podcast: UK-centric, pro cycling focused. Really good coverage of the major races from three well known journalists.
Cycling Tips Podcast: more about gear and cycling culture but still a bit of racing. Professionally produced and lots of good content.
The Paceline Podcast: all cycling culture, gear, fitness coaching. Rambling and a bit boring sometimes but still pretty good.

Out of those four, the Velonews and Cycling Tips casts are probably my favorite.