There is a halacha that one is not allowed to interrupt once he has started Shemoneh Esrei until he has finished, with various exceptions (O.C. 104).
Let's say I take my three steps forward and then realize that the guy about to start davening behind me is the world's slowest davener and I will be stuck here until Moshiach comes (speedily in our days amen!) if I don't relocate. Can I relocate at this point?
When am I considered to have started? After I take three steps forward? After I say Hashem sefasai? After I say baruch of the first beracha?