Does anyone have any tips on resetting our Visas when travelling over the permitted time in Europe under the Schengen Agreement?

I am hoping others can learn from my mistake.

My friend and I have booked flights from MEL to MAD 22/3/16 returning 6/7/16 ATH to MEL.

Due to ignorance, we did not realise we can’t stay in Europe for longer than 90 days without breaching the Schengen Agreement rules.

Does anyone have any advice as to how we can get around this, legally as we will be away for 105 days?

We will be walking the El Camino in Spain, then planned to travel through Portugal into Italy and then visit the Greek Islands via Athens, then home.

Can we leave the Schengen zone for 15 days, at some point and then come back to continue our journey?

Some articles say the 90 days starts at day of arrival, and leaving for 15 days wont make any difference.

Any advice gratefully accepted