Usually tempos are considered 30 sec slower than 5k and fartleks are 5 sec faster than 5k. Fartleks i would be doing 3 on and 2 off for 6 reps. Should I go at a tempo pace for them or will that be too taxing? Was thinking possible tempo pace or half marathon pace. Same with tempos? Thanks! True goal is to really build a good base over these 2 months
thats not a fartlek
A key characteristic of a fartlek session is not paying too much attention to your pace.
Particularly during a base phase, I wouldn't stress paces at all. Run the on-parts by feel, and aim for comfortably hard.
With regards to the specifics you mentioned:
I'll assume we're going by Jack Daniels' definition that tempo = LT = around 60 min race pace. Also, since you're saying that tempo pace is around 30 seconds slower than 5k pace, I'm guessing you're around 17 minute 5k pace.
If that's the case, I think tempo pace is quite reasonable for a 6 x (3 min off / 2 off). But it all comes down to what you're doing the rest of the week.
Regardless, like I said, I would worry less about pace and just run to feel at this stage.
Building a base? Just run baby. If you feel like hitting the throttle during a run, do it! Save the "structured" stuff for two months from now.