I'm happy to report @schoen 's directions worked well and quickly got cerbot running. I skipped a few steps since I already had certificates. For the sake of future readers, here's the complete list of commands I ran:
apt-get update
apt-get upgrade
apt-get install software-properties-common
add-apt-repository ppa:certbot/certbot
apt-get update
apt-get upgrade
apt-get install python-certbot-apachecertbot renew --dry-run
certbot renew
And then I edited the cron file to say certbot renew instead of letsencrypt renew