well if you insist on using runas inside of your script (going against what most admins on here would recommend), then i would say take a look at the runas program that can be found at the link below. Lets you put the password as a command line switch so that you dont have to use sendkeys. It also has a handy wait switch that will make it wait until the program called is completed before continuing.

http://www.anbsoftware.co.uk/netadmin/runas/