Over time the fluid gets contaminated with moisture, dirt, air and theres every likelihood it hasn't been touched for a long while. Do that first as its such a small job and just might fix everything. Symptoms of the clutch needing replacement are it getting heavy and ultimately, slipping. The linkage can get vague - mines rubbish at the moment. Theres a nylon bush below the lever that wears and causes problems getting reverse (search for other threads) or lack of lubrication and wear in the linkage mechanism on top of the gearbox that the cables from the lever connect into.
When its running everything is spinning round, synchros accelerating etc, so there is less resistance when the engine is off. But again, what you describe does throw some suspicion on the condition of the clutch fluid. Perhaps the slave is leaking slightly. Try pumping the clutch a few times before selecting gears with the engine running - same principle as pumping brakes - might improve it if the fluid s contaminated. DON'T BUY A CLUTCH YET!!