I think the answer depends a bit on how your cycles are and what your TTC/fertility status is. We don't own a car (on purpose), and biking is usually my primary form of transport. I had assumed that would be true while TTC as well, as I'd always heard you could keep up the same level of activity you usually did.
But after my first two cycles TTC (which followed almost a year of acupuncture to get me ovulating and fix my cycles) my acupuncturist more or less laid down the law and ordered me off the bike during the TWW. Her concern was that my uterus was poorly positioned, and "weak" (whatever that means in eastern-medicine-speak) and that extra "jostling" would hinder implantation. She also had concerns about certain points in my legs that biking would effect.
She hadn't mentioned it until after my first two cycles. At that point, I used something like Lex's regret logic, and begrudgingly got off the bike during the TWW. I've been off my bike for the whole pregnancy as well. That said, I got the feeling my acupuncturist's biking ban was coming from what she knew specifically about my body, so it was easier to take the advice, and she had also worked a miracle with my cycles, so I did trust her.
I will say though, that as happy as I am to be expecting our second, I will be beyond thrilled to get back on that bike in several months. And in terms of "giving things up" while TTC (which is to some extent inevitable), the freedom afforded by biking felt the hardest. But in terms of your health and sanity, it really might not make sense yet for you to give up running, at least on some level, especially if your cycles are great, your uterine position is good, and you have no reason to expect any problems. After a few cycles, it might feel different.
I'm Lyn (32) wife to Gail (38) Mama to Leigh (born 6/06 ,via Gail) and Ira (born 5/09, via me)