I agree, focus on one for now, one that will meet your needs and local airport. Why spread things around if you don't have to?
I would ditch Delta, award availability is there and earning miles is easy but it's expensive in miles to redeem a ticket. And F is usually booked out by paying customers or non-revs, so watch out if you go with Delta. I'd take one or two more flights with Delta to get up over 100k miles and then use them.
Whichever you go with make sure to get a CC that pays miles toward it, as well as find the best option for hotel and rental car. Stay focused and you'll do great.
I'd agree with the comment about 30% leading to 60%...my first job had 0% travel and I ended up being gone about 2 weeks a month for nearly a year. It was great for miles but stank based on what I expected from the job.