So I wouldn't call 20+ hours short for a guild questline. Most singleplayer games don't have half of that game length in their main campaign.
Are you talking specifically on the TG quest line, or are you also talking about the radiant quests that you get from two specific people within the guild?
It didn't take me too long to go through it, and I did not fast travel (though I used the carriages to go between cities). Overall, I enjoyed the TG questline, but I dislike that there is forced combat in it. If I wanted combat, I'd join the Companions. I am a thief, in the thieves guild, I want to steal things. The radiant quests are great, though I wish you could pick which city to hit (that got ridiculously annoying), but the story just didn't settle well with me. I liked the outcome, I enjoyed the last mission, and I had fun most of the time, but it just didn't seem very TG to me.
I will say that, to me, it was the longest of the 3 factions that I have played (I have not yet dome DB).