及ばずながら ……
Oyobazunagara …
However poor my efforts may be …

People are subjected to all sorts of requests and demands in the business world, some bigger than others. When you have been asked to take on some particularly important task or a major job, you may need to express your willingness to do so in a formal and self-deprecating manner. The same applies if you’re given an important new assignment or a promotion. On all these occasions, modesty first:

及ばずながら、ご期待にそえますよう尽力いたします。
Oyobazunagara, gokitai ni soemasu you jinryoku itashimasu.
However poor my efforts may be, I’ll try to live up to your expectations.

or

微力ながらお役に立ちたいと存じます。
Biryoku nagara oyaku ni tachitai to zonjimasu.
Meager as my talents may be, I’ll be glad to help out.

If you really want to get humble about it, you could trot out one of these:

至らない点もあるかと存じますが、全力を尽くす所存です。
Itaranai ten mo aru ka to zonjimasu ga, zenryoku o tsukusu shozon desu.
I’m afraid I may not be equal to the job in some respects, but I intend to give my all.

or

行き届かぬところもあるかと思いますが、私のようなものでよろしければお力に ……
Yukitodokanu tokoro mo aru ka to omoimasu ga, watashi no you na mono de yoroshikereba ochikara ni …
If you’re sure that, with all my shortcomings, I’m the person for the job, then I’ll do my best.

Hokey as they may sound, the examples above are for use in formal situations. Nobody talks that way under normal circumstances. To convey your enthusiastic consent in a more casual situation, you could use any of the following:

私でよければ、喜んで ……
Watashi de yokereba, yorokonde …
If I’m really the one you want, I’ll be glad to.

お安いご用ですよ。
Oyasui goyou desu yo.
No trouble at all.

そういうことでしたら、ぜひお任せください。
So iu koto deshitara, zehi omakase kudasai.
If that’s all it is, by all means, leave it to me.

いつでもおっしゃってください。
Itsudemo osshatte kudasai.
Certainly, anytime at all.

– Source: A handbook of common Japanese phrases –