给你二段老外在英文网站上的解答：

1 If somebody were to get up from his seat and say either of these sentences to others who were present, there would be no difference:

A: This has been a very informative meeting.
B: You're right. Well, I must go. / I must be going. I have to pick up my kids.

If the speaker is making a comment about a routine or a scheduled event in the future, only "I must go" would be appropriate:

A: But I thought you like your job.
B: I do, but I must go to every marketing meeting, and that really takes a lot of time out of my workday.

A: I hear your brother's getting married again. Are you going to the wedding?
B: Yeah, he's getting married -- for the fourth time! I'd really rather miss the wedding, but I must go or he'll never forgive me. 

2 "I must go" is a bit more abrupt, terse. "I must be going" is a bit more soft and could imply that it's some external force that obliges you to go......obviously depends on the context.

你可以看出，在表示“我得走了”这个意思时，有人认为二者没有区别，有人认为must be going要委婉一些，暗示必须离开是外因决定的， 而不是主观故意。