Unfortunately, ACS certificates are generated for a Life span of 1 year only. This cannot be changed or extended. You will have to generate another certificate after 1 year. The one year validity of the certificate is fixed to ensure greater security.
We generally buy in certificates fro www.rapidssl.com as they are pretty cheap ($316 for a 5 year cert) and the purchase process is painless, all web based and takes about 20 minutes to get your cert.
You can create your own self-signed certificates using IIS 6.0 resource kit.
There is information on Tech Republic - look for article on replacing LEAP with PEAP.