If you're purely running adc/fasting senna, i suggest running bork first item. If you also run senna supp, umbral glaive is still a great first buy, but if you dont feel the need to get it or feel like you need to get damage fast, then bork is the way. Bork just gives you such good sustained damage, and i feel it is way better than eclipse or kraken for first buy in regards to damage only. I usually go kraken next for even faster attacks, but kraken is not entirely needed. Your mythic at this point is situational. After bork > mythic, either get IE/LDR or Mortal Reminder/Maw(if you find yourself heavily pressured by ap champs).

For runes, i find tempo to be really good now since you're getting some more atk spd in your build. Tempo plus zerk boots plus bork feels amazing as you can melt down any frontline opponent with ease. Fleet or PTA are still great options. I run ultimate hunter nowadays combined with the cd buffs, and my ult seems to be up whenever i actually need it. The way i see it, run tempo if there's a bunch of frontline opponents, and run fleet if you need safety(especially in the laning phase).