What is a spiritual mentor/spiritual guru?
The path to awakening isn't available to everyone. It is only available to people who are ready for this experience. The way we are looking for a spiritual guide, sometimes are spiritual guide is also finding us. We need someone to help us connect with the divine, someone who is able to show us what its like to have an extraordinary outer body experience.
It is also believed the spiritual initiation by the mentors is only available to those who are actively searching for it and humbly ask for it. It isn't available to a man of ego or ill feelings.
Can one not pursue the path of spirituality without a Spiritual Mentor?
You should certainly begin your spiritual journey with meditation by yourself. As you progress, you should pray to god about hand-holding you and get you connected with your spiritual mentor. If you continue with determination and belief you should certainly find your spiritual mentor.
Awakening isn't a process of a day, a month or a year. There are yogis who have meditated 18 hours a day for 20 years to reach a higher state. Awakening is sometimes a process of multiple lives. It is believed that Christ was the spiritual mentor of John the Baptist and when he took birth on earth, they planned it the other way around. The spiritual journey between a mentor and disciples is sometimes a process that last multiple lives before they reach the state of immortality.