drills vs yardage

[bnichols4]

What are the best drills and/or combination of drills and full stroke yardage I can use to improve my long distance swimming? If I have 3 days a week to swim should I committ one or two of those days a week to only TI drills and the other day(s) to full stroke yardage. Or should I mix it up each day between TI and full strok? Any ideas or what has wroked well in the past?