The first time I hiked the Snowfield, when I was 14, I thought I was going to die. The sun was roasting, my legs felt like they weighed 50 pounds each, and no end was in sight. I finally dragged myself up to helicopter pad and collapsed in the dirt, while a six year old child laughed and frolicked past.
I've spent a many a day since then on the Muir Snowfield, and have found that how enjoyable the hike is will depend completely on conditions. Try to avoid the "Snowfield" when it's more of an ice sheet late in the year. And whiteouts are just not fun.
It's a fine hike for n00bs, though the altitude may be a problem. I'm sure your gf will keep up, and if not, you can always short-rope her to the RMI cattletrain .