Congrats! It's refreshing, isn't it? I try to commute one day a week and it definitely makes my day nice. As for co-workers not seeing you in spandex...I have found two solutions...

1. Get in eary before everyone
2. Get a skirt!

I like number 2 (although, I shower at work when I commute so #1 works well for me). There are a few companies that make skirts for riding and I find they work well. I feel much less self-conscious walking in/out with a skirt on than just spandex. I seriously feel like these skirts were made for commuters or people running errands!