Half marathon...?

Ok, so I'm signed up to participate in a half marathon tomorrow.  I do it every year and several family members participate, too.  Some years I run, other times I walk. Well, I'm 7w5d pregnant and not sure if I should even walk it or not.  After my 2nd MC last Sept, I quit running and am now only doing yoga & walking.  I usually walk 10,000 steps daily and I'm still very active (the half marathon will put me at about 35,000 steps tomorrow.).  I feel fine (aside from occasional headache/nausea) and my OB says this looks like a healthy, viable pregnancy and I can keep up with my usual activities.  I'm still a little worried - 13.1 miles is a long distance and even walking can be exhausting by the end.  On the other hand, if I don't participate, I am worried that my family memebers might wonder/guess what's up and I'm just not ready to announce yet.