Long burn times at low fire and low temp are what you WANT in a modulating condensing boiler.
When there is no flow on the system the boiler temperature will fall to about room temp, so 68-69F measured at both the input and output would be normal several hours after the last burn. On a new call for heat after a long stagnation the output temp should rise to whatever the outdoor reset is calling for within a few minutes, but the input temp will remain pretty low until the system water has gone full-circuit.
On a high water-volume or high thermal-mass system this can go on for quite some time, so if you're starting out with all the radiators at 68F, and the outdoor reset is calling for 150F water it can end up stressing the boiler. This can be avoided by not using deep over night setbacks at the thermostat, or just running the system continuosly, so that the return water is always tracking at a reasonable delta from the output temp.
Adjusting the outdoor reset curve to where it runs almost continuously but still manages to keep up with the heating load is the way max out the efficiency of these things. A right-sized boiler for the peak load will run VERY long burns, nearly constantly during mid-winter weather once it's dialed-in. Lowering the outdoor reset curve also limits the cold-start delta-T issue. If you tweak the the curves close enough recovery from setback is pretty slow, but running it continously the fuel use is the same or less than when using a deep overnight setback strategy, due to the greater condensing efficiency and near-zero cycling losses.
The best way to kill efficiency (and the boiler) is to have it doing fifteen 1-minute burns per hour at a high output temp with really big delta-T.