Thinking of moving my 401k to a Bond index.

(50m) This years Bull Market has got me back on track with my 401k, so I want to keep my gains. I'm currently in one of Fidelity's Blended Funds, but I'm considering moving to the FXNAX Bond Fund because this Market can't keep going up forever. Or am I just over thinking it? *Edit - Thanks for the responses, I appreciate the education! Also, I now realize I poorly worded this post. I meant to say I want to this TEMPORARILY for the next year or 2. Sorry, that was probably some important informati