Do your eight days include travel days? That is, do you land on Day One and leave on Day Eight? If that's the case, then you really only have six full days - you'll land likely mid-morning on Day One and be jetlagged (although I've found the farther east I travel, the easier the jetlag is), can explore Days Two-Seven, and then leave Day Eight. Not a ton of time. Are you flying in and out of Warsaw?
So, the challenge here is that, while Warsaw-Krakow is an easy, 2h15m (give or take a few) ride with no transfers, and Warsaw-Lublin is an easy 2.5-3.5 hour ride, Krakow-Lublin is an interminable slog. It was the worst train ride I've ever had to take, made worse because there is NO reason why it needs to be as long as it is. It is 5.5-6.5 HOURS. HOWEVER - Upon further examination, some trains on this route also stop at Krasnik and/or Zaklikow, towns you'll need to get to to get to Goscieradow. So that might make it worth it.
The challenge here is going to be fitting everything in. Only you can answer this: What do you most want to see? What are your priorities on this trip? Are you willing to sacrifice a day in Krakow or Warsaw to get to Goscieradow? If yes, then here's my suggestion:
Day One: Land Warsaw, get to the train station (Centralna), catch the next train to Krakow. There's one every hour except the noon hour. Let's say you get unlucky and have to catch the 1330; you are still in Krakow by 1546. Check in to your hotel, walk around the Main Square, have an early dinner, go to bed.
Days Two-Four: Three full days in Krakow, since you noted this will be where you spend the majority of your time.
Day Five: DOUBLE CHECK THESE TIMES. I don't know your exact dates, but plugged in 10 June. Leave Krakow at 800, arrive Zaklikow 1229. Yes, the Krakow-Lublin train. Yes, you will stop every 20 minutes. From Zaklikow, you can taxi twenty minutes to Goscieradow. Arrive by 1300. Have lunch, walk around. Taxi back to Zaklikow to catch the 1711 train to Lublin, arriving 1826. Check into hotel, have dinner. Sunset won't be until 2045 or so. I spent three wonderful summers in Lublin, so I am biased, but I think it's a wonderful town. If you had more time, I'd suggest you stay two nights. Alas, you don't, so -
Day Six: Train to Warsaw. Explore Warsaw.
Day Seven: Warsaw.
Day Eight: Fly Home.
Hope this helps!