I am doing similar stuff using a cheap 10yr old starboard allstar 12'6x26 i got a few months back. I started doing 6km paddles but now recently have done 2km quicker, 1km recovery / slower, another 2km back & then the final 1km at a pace however I'm feeling.

I'm assuming you are on the sea. So it depends on all the factors (wind, tide/ current, & swell/ chop). You should be faster than me with a 14ft but in general my speeds are around 7km/h.

I tend to go along the coast into the wind & back so the impact of the wind & tide should just about cancel each other out. Here's some that I've done.

10km @7.2km/h (friend with me on a 14ftx 23 said I was going some)
6km anything between 6km/h & 7.6km/h most around 7 or 7.3km/h
2km @ best is 8.2km/h (woohoo) & most about 7.5km/h

Conditions play the biggest factor. 8km/h on a 14ft should be achievable but best to do it in both directions (into wind & back) & take an average for a truer pace.

I've been doing it for about 4 months 2 to 3 times a week (it's been flat) & when I got out in decent surf conditions you can tell its helped my stamina & paddle power. I did 3 x 2hr sessions with no muscle cramps & without being out of breath or feeling knackered

(I'm 47 & 85kg)

@'theSeb' . Check out 'Sup My Race' on Facebook. People of varying abilities post their workouts (from around the world) with stats and data . I found it really useful (and interesting) to compare and check what others are up to .