I agree that something that isnt insanely long would be a bit easier to get in to or to give up if I didnt like it ultimately
Something with good plot, character development, humour and action... 26 episodes? There are very few shows that fulfill that criterion.
Code Geass (S1+2) is 52 episodes long and has lots of action, humour, good characterisation and awesome plot.
Eureka 7 is 50 episodes but is really very good (even though mecha is NOT my genre of interest).
Hikaru no Go is 75 episodes and has everything except romance. Is also incredibly easy to get into and not as repetitive as other competition anime.
Toradora! is not action, but has a good plot, character development, romance and humour and is 25 episodes.
Ouran High School Host Club is hilarious, 26 episodes, not much character development or action... but again, hilarious and awesome.
Others have already recommended
FMA and
Cowboy Bebop, and if I include
Darker than Black (S1+2) we have virtually the entire BONES list here.
Most shounen series are action-based with humour, but don't have great characterisation. Most shoujo series are romantic and have good characterisation, but no action at all. I watch both, and it's hard to find something with overlap. Bakuman (manga from Shounen Jump) is one of them, but it's only just being animated this season, and I assume that you want to marathon anime.