Club paddles are typically 15 - 25 km over a day, estimated distances are usually indicated in the information on the paddle when it is posted for participants.
It is also wise to consider the expected conditions which will effect the fitness needed to participate in the paddle. 15 km on a flat calm day is easy for most but 15km into a head wind of 10 knots may be a different undertaking. This is one reason why trips are graded (0,1,2 or3) and this should be your guide as to the level of fitness ( & skills) required.
Paddle fitness comes from practice, the more often you can paddle the quicker you will progress your ability to cover longer distances with safety and increase your speed.
Entry level paddles (grade 0) require you to be able to paddle around 15km at a resonable pace, we work on around 5kmh as an average pace. Level 3 is "up to 25km at an average of 6kmh with winds up to 20 knots" but often the distance is greater in a single day.
If you are a novice you should come on the regular monthly paddles and regular training paddles, where you will be able to improve your fitness under expert guidance and soon find yourself able to undertake more challengng journeys.
Dont worry about being left behind, the group can only ever paddle at the pace of the slowest kayak.