Two-part sitcoms are usually failures, because “brevity is the soul of wit.” Stretching out gags often makes them less funny rather than more funny. (The closest “Seinfeld” came to a good two-parter was “The Pilot” where a huge chunk of the hour was the eponymous, metafictional show-within-the-show.)
“Fun Run” demonstrates how you do it though: by giving the two parts different (albeit related) storylines. The first half is about Michael dealing with the blowback from hitting Meredith (declaring the office “cursed”) and the second half is about the Michael Scott's Dunder Mifflin Scranton Meredith Palmer Memorial Celebrity Rabies Awareness Pro-am Fun Run Race For The Cure. Woven through this episode is the fallout from the season 3 cliffhanger (Pam and Jim getting together, Ryan going to NYC, Jan moving in with Michael).
Every line in this episode is golden.
Ryan: Did this happen on company property?
Michael Scott: It was on company property, with company property. So, double jeopardy, we're fine.
Ryan: I don't think-- I don't think you understand how jeopardy works.
Michael Scott: Oh, I'm sorry. What is, 'we're fine'?