I'm assuming you're doing a cruise. If so, it's much easier to get to Mendenhall Glacier when in Juneau. There's a bus that runs on a regular basis - cost is $16 per person roundtrip to the visitor center and takes about 15 minutes to get there from town. Walking to the glacier is not strenuous and affords a wonderful opportunity to see a glacier up close and personal.
You can get more information here; Tongass Mendenhall Glacier Visitor Center