score:7
First of all, both Hamar and Lillehammer are located at the Lake MjΓΈsa, so there will be no net elevation change along the way. The pilgrim's path does not follow the shore though, so there are some light to moderate inclines along the way, but always accompanied with following descents.
Except from telling you that, noone here will be able to tell you which pace you prefer or can manage. Even being less than in 'decent shape', hiking 25-30km a day should IMHO be easily feasible in such light terrain.
The 'official' information page on this section of the pilgrim's road suggests that you use four days,