to get better/faster/fitter you need to do lots of walking... use the stair not the lift, walk don't drive if you are going a mile or 2, or park a mile or 2 from where you are going.
Wear good solid footwear and comfy clothes. Sort your feet out - short toe nails, good socks, be aware of rubbing - if you blister it will slow you down/stop you "training"....
Focus on walking - sounds daft, we can all walk can't we? No, some people amble and others walk. Get your head up, look where you are going, swing your arms, walk with purpose. Find some local trails, hilly if possible and get out there and walk them - take a water bottle and an apple or two with you and enjoy being outside.
I bet you will get to over 3mph on the day.
No rocket science here... but hope it helps. Good luck and Enjoy. 